ESTRO 2021 Abstract Book

S424

ESTRO 2021

Conclusion There was a trend towards improved image quality/reduced bowel motion artefact with both IM and IV buscopan. Buscopan was well tolerated by patients and its use within an abdominal/pelvic SABR workflow appeared to be feasible. The clinical impact of buscopan in reducing bowel motion artefact/improving radiotherapy image quality, for example in an adaptive SABR workflow, should be investigated. OC-0537 Interfractional changes of rectum and bladder effect on uterus position in cervix external beam RT Y.M. Tsang 1 , E. Sulaili 1 , A. Steward-lord 2 , D. Megias 1 , H. Tharmalingam 3 , P. Hoskin 3 1 Mount Vernon Cancer Centre, Radiotherapy, Northwood, United Kingdom; 2 London South Bank University, Institute of Health and Social Care, London, United Kingdom; 3 Mount Vernon Cancer Centre, Clinical Oncology, Northwood, United Kingdom Purpose or Objective To investigate the influence of interfractional position and volumetric changes of the bladder and rectum on uterus positional displacement during external beam radiotherapy (EBRT) of cervical cancer. Materials and Methods In a single centre retrospective cohort study, measurements of interfractional rectal positional and volumetric changes, bladder volumetric changes and tip of uterus (TOU) positional changes were carried out for 14 patients that underwent EBRT for cervical tumours. Measurements were carried out on initial planning computer tomography (CT) scans and on daily cone beam computer tomography (CBCT) scans. Rectum volumetric changes were determined by measuring anterior-posterior (AP) rectal diameter at S5 vertebrae level. Rectum positional changes were determined by measure of posterior rectal wall distance from S5 vertebrae. Bladder volumetric changes were determined via measure of AP bladder diameter and superior-inferior (SI) bladder diameter. TOU displacement was determined by measure of difference in TOU distance from the pelvic inlet plane and from the S1 vertebrae at planning CT scans and daily CBCT scans.